1. Field of the Invention
This invention relates to certain liquid adhesive and sealant compositions which, upon cure, have improved structural strength properties at elevated temperatures and improved resistance to thermal degradation.
2. Prior Art
Adhesive and sealant compositions based on cyanoacrylate monomers are known in the art. Typical examples of patents relating to cyanoacrylate adhesives are U.S. Pat. Nos. 2,784,215 to Joyner, 2,794,788 to Coover et al., and British Pat. No. 1,196,069 to O'Sullivan. Cyanoacrylate adhesive compositions are extremely sensitive, and great care must be exercised in their formulation. Cure (polymerization) is generally considered to be initiated by an anionic mechanism, with water being a sufficiently strong base to initiate the cure under most circumstances. The adhesives remain shelf-stable items of commerce as long as they are suitably packaged, but when placed on a substrate to be bonded and exposed to atmospheric and surface moisture, cure generally is instituted in a relatively short period of time, generally less than one (1) minute and on many surfaces, within a matter of seconds. This exceptional cure speed offers numerous advantages, particularly to those who use adhesive bonding in production line applications. However, a major shortcoming which has heretofore limited the areas of applicability of cyanoacrylate adhesives has been the relatively low thermal resistance of the cured bonds. Bonded assemblies frequently are exposed to continuous operating temperatures substantially above normal room temperature, and adhesive assemblies must retain reasonable strength for substantial periods of time at these elevated temperatures to retain their usefulness.
In addition to strength retention by the adhesive at elevated temperatures (i.e., hot strength), the adhesive bonds must not be unduly affected by continuous or repeated exposure to elevated temperatures (resistance to heat degredation). In the prior art it has not been possible to prepare a cyanoacrylate adhesive composition which produced substantial cross-linking on cure, even when ostensively di-functional cyanoacrylate monomers were used. Further, because of the extreme reactivity of the cyanoacrylate monomer, there have been substantial limitations upon addition of other ingredients, such as cross-linking agents or co-monomers, in order to improve the above-described thermal properties.
In U.S. Pat. No. 3,832,334, the problem of thermal resistance was solved by the addition of maleic anhydrides and their substitution products. Additionally, it has been known in the prior art to include cross-linking agents such as allyl-2-cyanoacrylate or polymerizable acrylate esters to improve thermal properties. However, none of the prior art has shown the improved thermal properties at 250.degree. F. that the instant invention is able to demonstrate. Thus, the need for such a useful adhesive is evident and would prove useful in many applications.